Field of the Invention
The present invention relates to a low-noise level landing apparatus and system for helicopters, which is intended to reduce noise generated during landing approach.
Noise generated from a helicopter during landing approach is mostly caused by the rotation of a main rotor. In particular, BVI (blade vortex interaction) noise is an impulsive noise which is caused by a sudden change of the blade air load during the interaction between the blade and previously shed blade tip vortex. The BVI noise, if generated, is louder than any other noise sources.
FIG. 8 is a graph showing an example of a BVI noise generating area with respect to the airspeed and descending speed of a helicopter. In this example, it is understood that the BVI noise is generated when the airspeed is in the range of 40 to 120 kt (knots) and the descending speed is in the range of 300 to 1,200 ft (feet)/minute, and that the BVI noise is loudest when the descending angle is in the range of 4.degree. to 6.degree..
For this reason, in order to minimize the BVI noise, it is desired that a helicopter should take a curved flight route by continuously changing the descending angle from the start of landing in order to avoid the BVI noise generating area.
However, it is very difficult for the pilot to bear the noise generating conditions shown in FIG. 8 in mind, to select a low-noise level flight route with the aid of instruments while manually controlling the helicopter. In particular, safety is the top priority during landing, and noise reduction is apt to be of secondary importance.
As an example of prior art, Japanese Patent No. 2,736,045 (Japanese Patent Application No. Hei 8-63558) has been disclosed, wherein a flight route is selected on the basis of a BVI noise generating area database obtained by previous analyses, flight tests and the like. However, the BVI noise generating area is subjected to the effects of meteorological conditions, in particular, the effect of wind, whereby low-noise level landing approach may not be accomplished in some cases.
FIG. 9 is a graph showing how the BVI noise generating area is changed. A flight route P1 is selected on the basis of the BVI noise generating area database. When meteorological conditions are no wind and the same as those under which the BVI noise generating area database is obtained, this route becomes a low-noise level landing route.
However, if the shape and position of the BVI noise generating area are changed by the effect of wind, the flight route P1 enters the BVI noise generating area, and is no longer a low-noise level route. As a matter of fact, a flight route P2 have been modified in accordance with the change of the area becomes a low-noise level landing route.